Experimental analysis of heat transfer enhancementin circular

International Journal of Mechanical Engineering and Technology ENGINEERING –
INTERNATIONAL JOURNAL OF MECHANICAL (IJMET), ISSN 0976
6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME
AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online) IJMET
Volume 3, Issue 3, September - December (2012), pp. 306-314
© IAEME: www.iaeme.com/ijmet.asp
Journal Impact Factor (2012): 3.8071 (Calculated by GISI)
©IAEME
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EXPERIMENTAL ANALYSIS OF HEAT TRANSFER ENHANCEMENT
IN CIRCULAR DOUBLE TUBE HEAT EXCHANGER USING INSERTS

Sunil Jamra
Lecturer, Mechnical Engg Deptt. , Sati Polytechnic College, Vidisha
S_jamra8593@yahoo.co.in
Pravin Kumar Singh
Assistant Professor, Mechnical Engg Deptt., B.U.I.T.,Bhopal
pravinbuit@rediffmail.com
Pankaj Dubey
Lecturer, Mechnical Engg Deptt., Sati Engg College, Vidisha
Pankajdubey450@gmail.com

ABSTRACT

In this study, heat transfer in a horizontal circular tube heat exchanger, with air as the working
fluid has been increased by means of rectangular inserts. In the experimental set up, cold air in
ambient condition was passed through the inner pipe while hot water was flowing through outer
tube. The Reynolds number of air varied from 15000 to 110000. The results were compared with
the plain tube with no inserts. The work included the determination of friction factor and heat
transfer coefficient for rectangular insert in both counter and parallel flow. In rectangular insert,
it was observed that the heat transfer coefficient varied from 0.9 to 1.9 times that of the smooth
tube value but the corresponding friction factor increased by 1 to 1.7 times that of the smooth
tube value. It was also observed that with an increase in Reynolds number (Re), the heat transfer
coefficient increased, where as the friction factor decreased.

Keywords: - Heat transfer Enhancement, rectangular inserts, heat transfer

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

NOMENCLATURE

A Heat transfer area (m2)
Cp Specific heat capacity (KJ/kg K)
D Equivalent hydraulic diameter (m)
F Friction factor
H Height of air channel (m)
h Convective heat transfer coefficient(W/m2 ̊C)
k Thermal conductivity (W/m ̊C)
m Mass flow rate of air (kg/s)
Nu Nusselt number
Pr Prandlt number
Q Heat transfer rate (KW)
Re Reynolds number
T Temperature (C)
∆Tm Logarithmic mean temperature difference (C)
V Average axial velocity (m/s)
µ Dynamic viscosity
v Kinematic viscosity
Subscripts
c cold fluid
h hot fluid
i inlet
o outlet
t total

1. INTRODUCTION

In recent years considerable emphasis has been placed on the development of heat
transfer enhancement techniques. Aydin Durmus et al, have augmented heat transfer rate in
concentric double pipe heat exchanger by passive method, snail type, swirl generator, which is
mounted at inlet and inside the inner pipe. They have reported that swirl flow enhances the heat
transfer mainly due to reduced boundary layer and increased resultant velocity. An augmentation
of up to 120% in Nusselt number was obtained in the swirl flow for counter flow. [1] Sarac and
Bali conducted experiments to investigate heat transfer and pressure drop characteristics of a
decaying swirl flow by the insertion of vortex generators in a horizontal pipe at Reynolds
numbers ranging from 5000 to 30000. They observed that the Nusselt number increase ranging
from 18% to 163% compared to smooth pipe. [6] S.N. Sarada et al investigated on enhancement
of turbulent flow heat transfer with mesh inserts in a horizontal tube under forced convection
with air flowing inside are with CFD analysis. [11] Ebru Kavak Akpinar, the study explored
the effect of different helical wires on the heat transfer, friction factor and dimensionless exergy
loss in a double concentric pipe heat exchanger. The key findings from the study may be
summarized as follows: Heat transfer rates increased with decreasing pitch and with increasing
helical number of the helical wires used in the experiments. The heat transfer rates in this heat

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exchanger increased up to 2.64 times with the help of the helical wires. [4] Thianponget al,
investigated compound heat transfer enhancement of a dimpled tube.The heat transfer and
friction factor increase with decreasing both of pitch ratio (PR) and twist ratio (y/w). Depending
on the pitch ratio and twist ratio, the heat transfer rate and friction factor in the dimpled tube with
twisted tape, are 1.66 to 3.03 and 5 to 6.31 times respectively of those in the plain tube twisted
tape swirl generator. [12]

2. Theoretical Analysis
The heat transferred to the cold fluid (i.e. air) is
Qc = mcCpc(Tco - Tci) = hcAi∆Tmi
The heat given by the hot fluid (i.e. water) is
Qh = mhCph(Thi - Tho) = hhAo∆Tmo
As usual, this heat may be expressed in terms of a heat transfer coefficient and tube logarithmic
mean temperature difference ∆Tm:
Qt = hA∆Tm
By equalizing the energy loss of the hot fluid and the energy received by the cold fluid,
convective heat transfer coefficients were deduced and Nusselt numbers were acquired as
follows [1]
Nu = hDH/k
For the hot and cold fluids, the Reynolds numbers are
Re = VDH/ⱱ

3. EXPERIMENTAL WORK

The apparatus consists of a blower unit fitted with a pipe, which is connected to the test
section located in horizontal orientation. The experimental study on passive heat transfer
augmentation using rectangular inserts was carried on in a circular double tube heat exchanger
having the specifications are Inner pipe ID = 25mm, Inner pipe OD=28mm, Outer pipe ID
=50mm, Outer pipe OD =56mm, Material of construction= GI (galvanized iron), Heat transfer
length= 2.50m, and Pressure tapping to pressure tapping length = 2.42m. The experimental set-
up used in this investigation is shown schematically in Fig. 1 In the experiment; precautions were
taken to prevent leakages in the system.
The inlet and outlet temperatures of the water and the air and of certain points along the outer
surface of the pipes were measured with a multi-channel temperature measurement unit in
conjunction with copper–constantan thermocouples. Pressure taps for measuring pressure losses
were provided at the inlet and outlet ends of the pipes, and they were connected to the two U
manometers, one of which was filled with water and used to measure the air-side pressure drop.
The other was used for the water-side pressure drop, and its manometer liquid was mercury. In
order to determine air flow rates, pressure taps were also mounted at Pitot tube, and they were
connected to another U manometer filled with water. The set-up also incorporated a throttling
valve and a Rotameter to control the water-flow rate, and necessary accessories (e.g. valves) to
change the flow mode. In each experiment run, the data for temperatures, flow rates, and fluid
pressure drops were recorded after steady-state was established. Reynolds numbers ranged from
2500 to 35000 and 10000 to 110000 for hot water and cold air, respectively. During the
experiments, the hot-water temperature and cold-air temperature varied from 30°C to 60°C and

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25°C to 50°C, respectively. All fluid properties were determined at the overall bulk mean
temperature.

Fig 1 Experimental Set up

Fig. 2 Rectangular Inserts

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Test Section
4. EXPERIMENTAL UNCERTAINTY

Experiments were conducted initially for plain tube without inserts difference indicated in U-
tube water manometer (with mass flow rate of air 0.0079 to 0.0437 kg/sec). The Nusselt number
obtained from experimental work is compared with the value obtained using Dittus-Boelter
equation (theoretical).The experimental uncertainty is found as 10% for Nusselt number.

5. RESULT & DISCUSSION

The most important aspects of this work were the extent of augmentation of heat transfer
and increase in friction factor associated with the introduction of rectangular insert into the air
flow. The turbulent flow and heat transfer in the inner tube mounted with rectangular insert in
(fig. 2) was measured with air as working fluid. Experimentation is performed with two cases
parallel flow and counter flow.
Validation of Plain Tube: - Preliminary experiments have been carried out on a plain tube in
both parallel and counter flow to check the facility performance and to verify the measuring
uncertainties. The experiments were carried out for a smooth tube to verify the validity. The
values of friction factor and Nusselt number obtained from the experiments were compared with
the values obtain from correlation of the Dittus - Boelter Equation for Nusselt number Nu =
0.023Re0.8 Pr0:4 …… [1] And Karman–Nikuradse equation for friction factor f = 0.046Re0:2 …….
[2] Fig 4 shows the comparison of the dittus-Boelter equation and plain tube values of Nusselt
number in counter and parallel flow it was found that the plain tube data agree reasonably well
with the values predicted by correlation.

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Validation of Plain tube
250

200 Plain Tube

Nusselt No.
150 counter flow

100 Plain Tube
Parallel Flow
50
Dituss- Boelter
0
0 50000 100000 150000
Reynolds No.
Fig 4 Comparison of experimental and theoretical values for plain tube.

6. RECTANGULAR INSERTS RESULT

Experiments were performed for parallel flow and counter flow results were compared to
those obtained from plain tube, In the case of parallel flow and counter flow for rectangular
insert the average increase in Nusselt number was 172% in comparison with that for the smooth
tube. The swirling flow gives higher values of Nusselt number than those for plain tube.
Rectangular insert increase in heat transfer rate ranges from 100 % to 172 % over the values
obtained for plain tube in the Reynolds number range of 10000 –110000. The increase in heat
transfer with rectangular insert is due to the higher swirl intensity imparted to the flow at the pipe
inlet. The swirling motion of the fluid (air) results in a pressure gradient being created in the
radial direction, thus affecting the boundary layer development.

300

250

200 Dittus-
Nusselt No.

Boelter
150
Plain Tube
100

50
Rectangular
0 Insert

0 50000 100000 150000
Reynolds No.

Fig. 5 Comparison for rectangular insert, plain tube and theoretical values for Counter flow

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250
Dittus-
200 Boelter
Plain Tube

Nusselt No.
150

Rectangular
100
Inserts

50

0
0 50000 100000 150000
Reynolds No.

Fig.6 Comparison for rectangular insert, plain tube and theoretical values for Parallel flow

The increased rate of heat transfer in such flows is a consequence of the reduced boundary layer
thickness and increased resultant velocity. From this figure 5 and 6 flow modes and increased
with Reynolds number. With the values obtained from parallel flow and counter flow
experimental data in inner pipe, the changes in Nusselt numbers with Reynolds numbers were
drawn at air side, as shown in Figure. In the figure 5 and 6, the Nusselt number was related as a
function of Reynolds number using the mass average velocity in the preliminary calculations.
The results obtained for plain tube and predicted values are also plotted for comparison in fig 5
and 6. It is seen that the effect of applying rectangular insert on the heat transfer rate is
significant for all Reynolds numbers used due to the induction of high reverse flow (turbulence)
and thin boundary layer. This technique results in an improvement of heat transfer rate over that
of the plain tube.

Friction factor: - Figure 7 shows the plots of Rectangular inserts values of the friction factor as
the function of Reynolds number for smooth plate and rectangular insert. It is clear that values of
friction factor drop proportionally as the Reynolds number increases due to the suppression of
viscous sub layer with increase in Reynolds number. The variation of pressure drop in terms of
friction factor across the test section as a function of Reynolds number for rectangular insert is
presented in fig. 7. It can be seen that the friction factor obtained from rectangular insert are in
similar trend and decrease with increasing Reynolds number. The increase in friction factor with
turbulent flow in general, is much higher than that with axial flow or plain tube flow. This is
because of the dissipation of the dynamic pressure of the fluid due to higher surface area and the
action caused by the reverse flow or turbulence. Moreover the pressure drop has a high
possibility of occurring by the interaction of the pressure forces with inertial forces in the
boundary layers it is seen from the figure that there is much reduction in the friction factor for
using rectangular insert.

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Comparison of friction Factor
theoret
0.014 ical
0.012

Fricton Factore
0.01 plain
0.008 tube
0.006
Rectang
0.004
ular
0.002
Inserts
0
0 50000 100000 150000
Reynolds No.

Fig 6 Comparison for rectangular insert, plain tube and theoretical values

CONCLUSIONS

Experimental investigations on enhancement of turbulent flow heat transfer with Rectangular
inserts in a horizontal tube under forced convection with air flowing inside are carried out. The
variations of temperatures, heat transfer coefficients, Nusselt number in the horizontal circular
tube fitted with rectangular inserts have been studied. The maximum increase in Nusselt number
of approximately 1.9 times was obtained through experimental investigation due to high
resistance offered to air flow for rectangular inserts. Pressure drop using rectangular inserts was
found to be maximum 1.7 times compared to that of plain tube.

REFERENCES

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